If you order a Jegs catalog (www.jegs.com), they have a ford section inside the engine section that has those GT-40P heads for a pretty good price. I think it's less than $300 each.
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Mustang Parts Specialties ripped me off, it could happen to you as well.
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT